It will sustain more than 1,000 jobs at Babcock in Devonport, and a further 300 at other companies in Plymouth. Such is the scale of the refit that another 700 jobs in the industrial supply chain across the UK will also be sustained.

HMS Vengeance is one of the UK's four Vanguard Class submarines, designed to carry the UK's Trident nuclear missiles. Together they form the backbone of the UK's continuous at-sea nuclear deterrent.

The upgrade is expected to take three-and-a-half years - clocking up a total of 2.5 million man-hours. It will involve a complete overhaul of equipment on the submarine as well as the installation of improved missile launch equipment and upgraded computer systems.

A new reactor core - the energy source that powers the 15,000-tonne vessel - will also be fitted, meaning that HMS Vengeance will be able to function until the end of her remaining operational life without having to refuel again.

Talking about the HMS Vengeance refit contract during his visit to Devonport Dockyard, the Defence Secretary said:

"As well as securing 2,000 UK jobs, this contract will ensure the nuclear deterrent submarine fleet can continue to operate safely and effectively to maintain a continuous at-sea deterrent. As we stabilise the Defence Budget we are increasingly able to commit to equipment projects to safeguard the UK's national security.

"Devonport Dockyard is at the heart of maintaining and supporting the Royal Navy and I am pleased that such a large number of jobs will be protected.

"Our White Paper published earlier this year said we would support key sovereign capabilities in British companies that help us to protect our national security and this contract with Babcock is evidence of that commitment."

Work on HMS Vengeance will begin in the next few weeks. She is the last of the four ballistic submarines to undergo a complete overhaul and refuel. The upgrade of her sister vessel HMS Vigilant was recently completed and she is due to depart Devonport for sea trials tomorrow, Tuesday 27 March.

The Royal Navy's newer Astute Class submarines are designed so that they never need to be refuelled.

Rear Admiral Simon Lister, Director of Submarines for the MOD, said:

"This contract marks the final phase of what has proved to be a very successful programme of refuelling our fleet of nuclear submarines. The highly sophisticated nature of the work involved in the deep maintenance of these magnificent vessels is testament to the experience and skills of the workforce here in Devonport and those in the supply chain across the UK."

Even though this is the last time a complete refit will be carried out, the submarine dockyard at Devonport will continue to carry out regular maintenance on all of the Vanguard Class to ensure they operate safely and effectively.